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Freezing can only work on hidden pages, currently there is no easy way
in webdriver that allows us to hide the page.

A new webdriver API is in the works to enable that, but meanwhile, for
all testing API, we have to couple hiding with freezing.

This CL does that and reenables the test with Network Service.

Bug: 837709, 848604
Cq-Include-Trybots: master.tryserver.chromium.linux:linux_mojo
Change-Id: Ib5c513aefb219cef38a563be81fe8abf4b354051
Reviewed-on: https://chromium-review.googlesource.com/1089354
Commit-Queue: Fadi Meawad <fmeawad@chromium.org>
Reviewed-by: Kenneth Russell <kbr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#565031}